It depends on the kind of ride.

My commute to work is about 2.5 miles, taking 20-25 minutes depending on traffic lights. It takes just as long to drive and find parking, and longer to take the bus. So for the sake of convenience, I'll definitely ride if the temperature is 40ºF, and will usually ride down to 30. It doesn't often get colder than that here. Combine cold with heavy rain and I'm more likely to drive.

I'm more of a fair-weather biker when it comes to "fun" rides, usually noping out if the expected high temperature is below 60ºF. I had too many hellish rides thanks to Raynaud's.

Of course, this year I'm thinking of doing an event in February so I better start getting on my bike despite the cold weather.